Cherry-pick #22438 to 7.x: Fix duplicated events in kubernetes autodiscover for pods with init or ephemeral containers

[jsoriano]

Cherry-pick of PR #22438 to 7.x branch. Original message:

What does this PR do?

Stop sending duplicated events for pods with init or ephemeral containers.

Why is it important?

Actually not so important because in principle duplicated events should generate the same configuration, and configurations are de-duplicated, but just in case these duplicated events have unexpected effects.

Also, an additional test is added to cover init containers.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Issue discovered while reviewing Autodiscover ephemeral containers in kubernetes #22389

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[elasticmachine]

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	16369
Skipped	1367
Total	17736

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #22492 opened]

• Start Time: 2020-11-09T13:15:24.596+0000

• Duration: 100 min 21 sec

Test stats 🧪

Test	Results
Failed	0
Passed	16369
Skipped	1367
Total	17736